Folic acid blinded trial in identical twins with fragile X syndrome.

Monozygous twin 14-year-old mentally retarded boys with the fragile X syndrome were treated either with 10 mg folic acid by mouth daily or with a placebo for three test periods of 3-month duration each in a blind study. For each twin, tests of cognitive functioning, reading, spelling, and math skills, and linguistic and perceptual skills were compared. Although there was considerable variation in performance on these tests during the two baseline periods, there were no observable beneficial effects of therapy. The routine use of folic acid in patients with established mental retardation and the fragile X syndrome is not indicated.